1. Field of the Invention
The present invention relates generally to soldering of metal surfaces without using a flux or other separate surface treatment. More particularly, the present invention relates to a solder which includes a reducing agent which preferentially reacts with and removes interfering oxides during the soldering process to enhance surface wetting and solderability.
2. Description of Related Art
In order to produce high quality solder joints, it is essential that the surfaces to be joined be free of contaminants. Many contaminants such as organic solvents and cleaning fluids can be removed by relatively simple cleaning processes. Other contaminants such as metal oxides are more difficult to remove. The presence of metal oxides on the surface to be soldered or in the solder itself prevents adequate wetting of the surface with solder and results in a poor bond. Such oxides are referred to herein as "interfering oxides". A variety of different flux materials has typically been used to remove oxides and other contaminants from the surface to be soldered. Fluxes may be incorporated directly into the solder or they may be applied separately to the surface prior to soldering.
A major problem with fluxes is they leave a residue which many times is difficult to remove from the surfaces of the soldered components. In addition, the removal of such residues often requires the use of chlorofluorocarbons which have an adverse effect on the environment and which will be banned from use in the near future. As a result, alternative soldering procedures have been sought in which high quality solder joints are produced without the use of flux.
One approach to fluxless soldering involves maintaining an atmosphere surrounding the components to be soldered which is conducive to solder bond formation. For example, U.S. Pat. No. 4,646,958 discloses a fluxless soldering process where the soldering components are maintained in an atmosphere comprising from about 0.1 to about 10 percent by volume silane gas. U.S. Pat. No. 4,937,006 discloses another fluxless soldering process where a hot reducing or non-reactive gas is directed at the solder mound. The heated gas has a momentum which is sufficient to disperse the contaminating oxide layer on the surface of the molten solder.
Another approach to providing a fluxless solder procedure involves adding different components to the solder in order to provide a composition with no char-forming constituents such as rosins. For example, U.S. Pat. No. 4,728,023 discloses a fluxless solder composition which is made up of finely divided solder metal and a fluorinated tertiary alkylamine which acts as a vehicle for the solder. During the soldering process, the surfaces to be soldered are placed in a chamber which contains boiling vapors of a second liquid fluorinated tertiary alkylamine having a lower boiling point than the fluorinated tertiary alkylamine in the solder composition. As a result, the fluorinated tertiary alkylamine in the solder dissolves in the lower boiling point fluorinated tertiary alkylamine and is removed during the process.
The above-described processes are exemplary of attempts to provide soldering processes which are not dependent upon fluxes which leave an undesirable residue. It would be desirable to provide an improved fluxless soldering process which does not require reactive gases. Further, it would be desirable to provide a fluxless soldering composition which includes additives that remove contaminating oxides during the soldering process without leaving a residue which must be cleaned or otherwise recovered during the soldering process.